The present invention relates to arrays and, in particular, to so-called Bessel-weighted arrays of electroacoustic actuators.
Loudspeakers of a loudspeaker array, such as, for example, a linear array or area array, may be controlled in different ways. EP 0034844 A1 discloses amplitude/phase weighting based on the functional values of the first type Bessel function with different orders.
A possible embodiment of such an array is shown in FIG. 11a. It consists of five individual loudspeakers which are referred to by 1, 2, 3, 4, 5 in correspondence with their arrangement in, for example, a linear array. The amplitude/phase weighting is illustrated in FIG. 11 to the left of the loudspeaker array 1100. The two outermost loudspeakers exhibit a weighting of 0.5 and the inner loudspeakers exhibit a weighting of 1, wherein one loudspeaker, i.e. loudspeaker 2, additionally exhibits a phase shift of 180°.
Compared to a single loudspeaker, such an array achieves a higher sound pressure level. Although the array has a larger radiation area than the single loudspeaker, the radiation characteristics hardly differ from each other.
For the linear loudspeaker array shown in FIG. 11a which consists of five active loudspeakers of the same type, the Bessel weighting provides for the amplitude ratio which is indicated to the left of the array 1100. The phase ratio is 0°:180°:0°:0°:0° of the individual loudspeakers relative to one another. FIG. 11b shows a connection of the loudspeakers to form a series connection. In particular, loudspeakers 2, 3, 4 are connected in series and these in turn are connected in series to a parallel connection of the two outer loudspeakers 1 and 5. Thus, the Bessel-like weighting necessitated for each loudspeaker may result due to the corresponding voltage drop.
Alternatively, the Bessel weighting may also be generated using a parallel connection consisting of several parallel branches (FIG. 11c). One of these parallel branches consists of a series connection of loudspeakers 1 and 5, the remaining parallel branches each contain an individual loudspeaker (2, 3, 4).
Of advantage with the connections in FIGS. 11b and 11c is the fact that the Bessel weighting necessitated may be realized just by suitably connecting the loudspeakers. The amplitudes are achieved by the parallel/series connection and the phases by a corresponding polarity of the loudspeakers among one another. In FIG. 11, this results from the fact that the polarity of loudspeaker 2 is opposite compared to the polarities of the other loudspeakers, i.e. the negative input of the loudspeaker is connected to the corresponding positive output of the loudspeaker amplifier which is not shown in FIG. 11.
However, overall impedance of the array is a problem of such a connection. When serially connecting the 5-Bessel array of FIG. 11b, the result is an overall impedance of the array corresponding to 3.5 times that of the individual loudspeakers. With a nominal impedance of the individual loudspeaker of 4Ω or 8ΩA, the overall impedance of the series connection correspondingly will be 14Ω and 28Ω, respectively. However, conventional audio amplifiers are optimized for nominal impedances of 4Ω to 8Ω. A considerably higher voltage amplification is necessitated for driving an impedance of 14Ω with the same electrical power like an impedance of 4Ω.
For a realization by means of a parallel connection in FIG. 11c, the impedance of the 5-Bessel array is reduced to 0.29 times that of the individual impedance. For an array of 4Ω or 8Ω loudspeakers, the overall impedance will consequently be 1.14Ω and 2.29Ω, respectively. Usually, this is considerably below the load impedances optimal for present/modern amplifiers. Too high a current is demanded of the amplifier, which may result in the destruction of devices.
For this reason, the Bessel weighting cannot be realized optimally using loudspeakers of conventional impedance such as, for example, 4Ω to 8Ω.
With regard to linear arrays having a greater number of loudspeakers, the number being greater than five, the overall impedance reaches an even smaller value with a parallel connection and, with a series connection, an even greater value when the same loudspeaker impedance is assumed.